A basic requirement for the operation of a radio system is that the connection between a transmitter and receiver is synchronized. In conventional radio technology, a PLL receiver (Phase Lock Loop) is employed to achieve and maintain synchronization. However, this method is not very useful in the CDMA system (Code Division Multiple Access).
In the CDMA method, a narrowband user data signal is multiplied to a relatively broad band by a spreading code having a considerably higher frequency than that of the data signal. The aim is to select spreading codes that are mutually substantially orthogonal, i.e. have minimal mutual correlation. Upon transmission, a broadband signal is multiplied, i.e. modulated, by a continuous carrier having a considerably higher frequency than that of the spreading code. Owing to orthogonal spreading codes, the signals of several users can be transmitted on the same carrier.
In a conventional CDMA receiver, the signal carrier is demodulated by multiplying the signal by the tuning frequency of the local oscillator of the receiver. The tuning frequency is intended to have the same frequency as the carrier employed in transmission. The data signal again is returned in the receiver to the original band by multiplying it again by the same spreading code as at the transmission stage.
However, there is usually frequency difference between the tuning frequency of a local oscillator and the carrier employed in transmission. It can be seen as a phase shift between the receiver and the spreading code employed in transmission. Without corrective measures, this phase shift increases the longer the connection between the transmitter and the receiver lasts. In a fading multi-path environment, the change caused by the difference between the transmission carrier and the receiver tuning frequency in the code phase is called a group shift. In a typical multi-path environment of a radio system this increasing group shift cannot be detected reliably with prior art measurements.